The invention is directed to an apparatus and method to measure and paper stock thickness in the forming section of a papermaking machine for monitoring de-watering efficiency.
In the manufacture of paper on papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric and water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The paper machine is, in essence, a giant de-watering, i.e, water removal, system. The largest amount of water is taken out in the forming section as the stock is de-watered from a consistency to 0.2%-11/2% solids to a web having a consistency of about 18%-25% solids. A typical forming section of a papermaking machine includes an endless traveling papermaking fabric or wire screen which travels over a series of water removal elements such as table rolls, foils, vacuum foils, and suction boxes. The stock is carried on the top surface of the papermaking fabric and is de-watered as the stock travels over the successive de-watering elements to form a sheet of paper. The amount of de-watering is directly proportional to the reduction of the paper stock thickness. Finally, the wet sheet is transferred to the press section of the papermaking machine where enough water is removed to form a sheet of paper with about 36%-44% solids. The various de-watering stations in the forming section have rated capabilities for de-watering. It is advantageous to be able to measure the actual de-watering occurring in the stock to determine if the dewatering element is performing according to its capabilities.
Various devices have been proposed for monitoring the de-watering or drainage efficiency of the papermaking fabric along the forming section. For example, it is know to use an ultrasonic meter to monitor the de-watering efficiency along the forming section. An ultrasonic transistor is placed at different positions under the papermaking fabric and a pulse of ultrasonic energy is reflected from the stock/air interface on top of the fabric. In this manner, information as to thickness of the stock, and hence the degree of de-watering, is obtained. However, since the ultrasonic meter measures the distance to the stock/air interface, there may be variations in the measurements which are not caused by changes in the moisture content. These variations may be due to disturbances in the surface, or due to air entrapment. To over come the deficiencies of the ultrasonic devices, Great Britain Patent No. 2,260,408 discloses a microwave moisture meter for monitoring the de-watering efficiency along the forming section. A microwave moisture meter is placed underneath the fabric, and energy is directed through the fabric and the stock by the meter. Modification of the microwave energy caused by the moisture content of the stock is monitored by the meter. It is said that this arrangement allows the moisture content to be measured substantially without having to make complicated or inconvenient allowances for variations in other parameters.
However, the problem with both the ultrasonic and microwave moisture meters is such meters are operated underneath the papermaking fabric in the forming section of the papermaking machine. This creates difficulty in the ease at which the de-watering measurements may be taken, and can effect the accuracy of the measurements as well.
In addition, it is known to use a device commonly referred to as a Gama- gauge to measure the de-watering efficiency at the various stations in the forming section. However, safety problems due to the radiation associated with these type of devices cannot be entirely ruled out. Also, these devices are relative sensitive which makes their transportation a problem. Other devices for measuring the moisture content of a moving sheet of paper during the manufacturing process of a papermaking machine are disclosed in U.S. Pat. Nos. 3,614,450 and 3,851,175. These devices employ moisture gauges having two detectors of different wavelengths. Typically, one wavelength is highly sensitive to the moisture and the other wavelength is relatively insensitive to the moisture. The ratio of the two signals is utilized to provide a signal representative of the absolute moisture content.
U.S. Pat. No. 3,847,730 discloses a similar system wherein maximum and minimum moisture signals are compared for determining moisture content in the manufacture of paper. U.S. Pat. No. 3,713,966 discloses a plurality of moisture gauges disposed across the width of the moving web which collectively indicate the moisture content of the web.
Accordingly, an important object of the present invention is to provide an apparatus and method for monitoring the de-watering efficiency of a forming section of a papermaking machine which are simple and reliable.
Another object of the present invention is to provide an apparatus and method for monitoring the de-watering efficiency of a forming section of a papermaking machine which does not require electrical devices or meters underneath the papermaking fabric on which the papermaking stock is carried.
Yet another object of the present invention is to provide an apparatus and method for monitoring the de-watering efficiency at various stations along the length of a forming section of a papermaking machine wherein the de-watering monitoring apparatus may be easily transported along the forming section for taking different measurements.